In restoration of a tooth damaged by caries, fracture, or the like, a restorative material called a composite resin, which is formed of a polymerizable composition (which is generally photocurable) in a paste form, is widely used because the restorative material allows a simple operation. In recent years, the polymerizable composition has been improved in mechanical strength and also in adhesive force for a tooth, and hence has been used for not only restoration of an anterior tooth portion, but also a posterior tooth portion, to which a high occlusal pressure is applied. The restoration of a tooth using the dental restorative material is generally performed by a method involving:                1) forming a cavity in a tooth damaged by caries or the like (hereinafter sometimes referred to as “tooth to be restored”) by cutting;        2) building up the polymerizable composition in the cavity through intermediation of an adhesive (in general, an adhesive called a bonding material is applied to the cavity, and as necessary, an air blow is performed, followed by photoirradiation to cover the cavity with the adhesive), followed by photocuring to seal the cavity; and        3) finally subjecting the cured body of the composite filling and restorative material with which the cavity is filled to shape modification and polishing.        
The restoration of a tooth requires satisfactory aesthetics in addition to operability, mechanical strength, and the like. A natural tooth is formed of dentin and enamel, and its color tone is almost entirely derived from the dentin, which is observed through the transparent enamel. It should be noted that the enamel increases from a tooth cervical portion toward an incisal portion, and the color tone (at least one kind of element selected from a hue, a chroma, and a lightness) differs from site to site. For example, the tooth cervical portion has a thick dentin layer, and hence is opaque and has high values for the lightness (light and shade of color) and the chroma (intensity of color) as compared to the incisal portion. In contrast, the incisal portion has a thin dentin layer and is almost entirely formed of the enamel, and hence has high transparency. As described above, the color tone of a tooth differs from site to site. Accordingly, in order to obtain high aesthetics in the restoration of a tooth, it is important to prepare a plurality of kinds of polymerizable compositions having different color tones, and to use one selected therefrom and having a color tone that best conforms to those of an actual tooth to be restored and teeth adjacent thereto (hereinafter sometimes referred to as “surroundings of the tooth to be restored”) (Non Patent Literature 1).
A dentist performs such color tone selection with a shade guide (color sample), which is a collection of respective cured body samples of prepared polymerizable compositions, by comparing the color tone of each of the samples to the color tone of the surroundings of the tooth to be restored, which is checked by looking into the mouth, and selecting the one considered to have the color tone closest to that of the surroundings of the tooth to be restored.
In addition, unless the damage of the tooth to be restored is light and the cavity is shallow, the conformation of the color tone is difficult to achieve by filling with a single kind of polymerizable composition. That is, when the cavity is deep (e.g., a class IV cavity), the color tone of the tooth is observed not merely as the color tone of a tooth surface portion (enamel portion) but in a gradation-rich state in which color tones, even including that of a deep portion (dentin portion) that can be seen through, are blended. Accordingly, such subtle color tone is reproduced by layered filling, which involves changing the color tone of the polymerizable composition to be filled at certain depth intervals. In general, a plurality of kinds of polymerizable compositions for a dentin portion restorative material, which reproduce the color tone of the dentin portion, are layered from the deepest portion (each layer is generally cured before further layering), and finally, a polymerizable composition for an enamel portion restorative material is layered as a surface layer portion (see, for example, Non Patent Literature 1 and Non Patent Literature 2).
Further, there is also a proposal of a material that is conformed to a natural tooth in terms of optical characteristics as well as color tone ((Patent Literature 1). With a focus on a light-diffusing property, it is reported that restoration with good aesthetics can be performed by using a material having a high light-diffusing property for a portion including a large amount of dentin and using a material having a low light-diffusing property and a high transmissivity for a portion including a large amount of enamel.
In addition, there is also a report of a dental restorative material having its refractive index adjusted. In Patent Literature 2, refractive indices of a dentin restorative material and an enamel restorative material are adjusted to be similar to each other. In Patent Literature 3, an enamel restorative material is adjusted to have a high refractive index in order to reproduce optical characteristics of a natural tooth.